The present invention relates to the generation of seismic energy in a marine environment via the use of seismic sources employing the abrupt release of a pressurized gas.
In marine seismic exploration, the term "marine" as used herein being intended to encompass any body of water regardless of its location or depth, a pulse of acoustic energy is released into the water at close intervals to generate acoustic waves that propagate through the water and into the earth's crust. The waves are reflected and refracted by subsurface formations and at interfaces therebetween, thereafter traveling back to receivers, at or closely beneath the earth's surface where the waves are converted into electrical signals and are recorded on instruments as digital data for subsequent processing followed by interpretation and analysis.
In recent years, the dominant marine seismic energy source has been the air gun, a device which releases high pressure air, typically at 2,000 to 6,000 psi, into the water to create the desired acoustic wave. A state-of-the art air gun design disclosing and claiming a unique 360 degrees post design is disclosed in U.S. Pat. No. 4,623,033 assigned to Geophysical Service Inc., although air guns are designed and manufactured by numerous vendors and the specific gun design utilized with the present invention is not critical to its operation. An air gun utilizing a dual chamber reciprocating shuttle design is disclosed in U.S. Pat. No. 4,211,300, assigned to Western Atlas International, Inc.
It is known in the art to dispose a plurality of air guns in an array for several purposes. For example, U.S. Pat. No. 4,108,272, assigned to Western Atlas International, Inc. discloses the concept of grouping three or more air guns tuned to different pulse frequencies to generate a simple, non-repetitive pulse train during a recording cycle. U.S. Pat. No. 4,648,479, assigned to Exxon Production Research Co., discloses a multi-chamber air gun wherein the exhaust port sizes and locations may be chosen so that air bubbles emanating therefrom will interact or "coalesce", the benefits of which will be hereinafter discussed; the use of guns timed to different frequencies is also disclosed. Another multi-chamber gun is disclosed in U.S. Pat. No. 4,381,044, also assigned to Exxon Production Research Co. U.S. Pat. Nos. 4,034,827, 4,047,591 and 4,719,987, assigned to the assignee of the present invention, disclose the use of arrays of multiple air guns and the '987 patent illustrates an actual physical arrangement of an air gun array as deployed in the prior art, wherein guns are arranged in groups of four of similar orientation and are linked together by flexible support means such as chains or cables. U.K. Patent Application No. GB2176605A, assigned to Exxon Production Research Co., discloses air guns deployed in a four-gun configuration similar to that disclosed in the aforementioned '987 patent and suggests that a group of three or more guns at critical intergun distances maximizes suppression of successive oscillations of the large bubble created by the coalescing smaller bubbles of the individual guns, thereby maximizing the "primary to bubble" ratio, the latter being defined as the amplitude ratio of the primary signal component of the seismic signal to the envelope of the accompanying successive components of the signal. The foregoing patent references Safer, M. H., "Efficient Design of Air Gun Arrays," Geophysical Prospecting 24, 773-787 (1975) regarding air gun spacing in arrays.
It is well known that in air gun arrays, groups or clusters of air guns deployed in close proximity to each other generate an acoustic pulse from the coalesced bubbles of the individual guns in which the primary to bubble ratio exceeds that of a pulse generated by a single gun of the same volume as that of the combined volumes of the clustered guns. See Giles, B. F., and Johnston, R. C., "System Approach to Air-Gun Array Design", Geophysical Prospecting 21, 77-101 (1973).
The relationship for air gun acoustic pressure output can be represented as follows: EQU P.sub.a =NKV.sup.1/3
Where:
P.sub.a =Acoustic pressure, bar-meters PA1 N=The number of air guns PA1 K=Constant for the particular air gun porting PA1 V=Air gun volume, cubic inches
Therefore, in order to alter (increase) acoustic pressure output using the same chamber pressure, one can either increase chamber volume of a gun (V), increase the gun (porting) size (K), or increase the number (N) of guns. Assume that one wishes to obtain the maximum acoustic output for a given air gun chamber volume. Further assume that one may utilize a 40 cubic inch chamber volume air gun manufactured by Geophysical Service Inc., assignee herein, and designated as a GSI Sleeve Gun I, ("K"=0.585) or a larger, 160 cubic inch chamber volume air gun of the same manufacture, designated as a GSI Sleeve Gun II ("K"=0.630). If the chamber volume "V" of the Sleeve Gun I is increased from 40 to 160 cubic inches, P.sub.a =3.17 bar-meters. If an unmodified Sleeve Gun II is employed, P.sub.a =3.42 bar-meters. If four (4) unmodified Sleeve Gun I's are employed, giving a total of 160 cubic inches of chamber volume, P.sub.a =8.00 bar-meters, proving the vastly greater efficiency of using many guns of smaller volume rather than a few larger guns, an approach followed by a few geophysical companies for some time.
The above result is modified somewhat by placing guns close enough to one another so that their individual bubbles coalesce into a larger bubble size. Such placement results in a reduction of the acoustic pressure radiated by the gun cluster, due to the interaction effects of each gun changing the ambient pressure of neighboring guns. This reduction is on the order of fifteen percent, which would reduce the output in the above example to approximately 6.80 bar-meters, still a sizeable increase over the option of using a single, larger gun of equal volume to the clustered, smaller gun.
An advantage of using multiple "coalesced" air guns is the gain in the higher frequencies, as a smaller air gun has a short pulse length (higher frequency content) than the signature of a larger air gun. By using multiple small guns, the small air gun pulse length (higher frequency content) is maintained while the larger air gun bubble pulse is achieved, providing the lower bubble frequencies. This characteristic has the effect of broadening the acoustic spectra without appreciably affecting the lower frequencies, which are primarily generated by the bubble oscillations.
A problem inherent in the close spacing of coalesced air guns, however, is damage to the guns, air hoses, electrical cables and associated mounting hardware (such as shackles and chains, etc.) due to the recoil of each individual air gun plus the pressure waves caused by each air gun acting on surrounding air guns. The continual movement of the reacting air guns can cause catostrophic damage if a chain should part or at the least causes rapid wear of the mounting hardware, thus changing the air gun cluster spacing, which in turn can lead to a change in the cluster's acoustic pulse "signature", changing the frequency spectra and lowering performance from the original design optimum.